Extrinsic semiconductors its definition and its types

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Saurabh Guptahttp://karekaise.in
नई तकनीक का आविष्कार, गैजेट्स, उपभोक्ता प्रौद्योगिकी और सॉफ्टवेयर के लिए आपका स्रोत. कंप्यूटर, स्मार्टफोन, इलेक्ट्रॉनिक गैजेट्स और इंटरनेट सामग्री पर नवीनतम रुझानों के लिए हमारी वेबसाइट देखें!

if you are a student of physics then you had definitely heard about semiconductors, now a days semiconductors are used every where like if you are reading this in your mobile or computer then its screen is also made up of semiconductors and now LED bulbs are also used everywhere in houses, in offices and in shops etc. Now semiconductors plays an important role in human life. In this article you will get complete information about Extrinsic Semiconductors.

What is Extrinsic Semiconductor

The electrical conductivity of intrinsic (pure) semiconductors increases with temperature even on reaching room temperature 27°C its conductivity is very poor. The addition of the certain impurities can increase the conductivity of the intrinsic (natural) semiconductors. Very small amount (in parts per million PPM) of impurity atoms can increase conductivity of intrinsic semiconductors many times.

eg:- (1atom of impurity in 10^8 pure atoms of Ge increases conductivity  of pure Ge 16 times).

“The semiconductor thus obtain is called Extrinsic semiconductor

The process of addition of impurities (intentionally) is called “Doping”  and the impurity atom is called ‘Dopant’. This semiconductor is also  known as doped semiconductor.

Doping can be done in such way:-

(i) heating pure semiconductor in atmosphere of impurity atom so that impurity atom diffuses in pure crystal.

(ii) bombarding pure crystal with ion of impurity atoms.

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Types of Extrinsic semiconductor

Extrinsic semiconductor can be of two types based on the the type of impurity atom:-

Extrinsic semiconductor types

1. N – type semiconductor:- pentavalent impurity atom.

2. P – type semiconductor:- trivalent impurity atom.

N – type semiconductor

Pentavalent impurity atoms like phosphorus, antimony ,arsenic replaces pure silicon (Si) or germanium (Ge) atoms in crystal. The ionization energy required to free fifth valence electron is very low contrary to that required in pure semiconductor. In pure semiconductors energy is required to break covalent bond.

Ge – 0.72 eV

Si – 1.1eV

the energy required to free fifth valence electron can be obtained at room temperature so this electron is set free and it goes to conduction band and it is available for electrical conduction. pentavalent impurity donates 1 electron per atom and hence they are called donor impurity.

The conductivity increases with doping and conductivity of N type do not depend on temperature. Note that some free electrons are also generated thermally due to breaking of few covalent bonds and an equal number of holes are also generated.

In N -type semiconductors electrons are majority charge carrier. N type semiconductors are electrically neutral, pentavalent impurity brings one extra electron but it also brings one extra proton.

P – type semiconductor

In p type semiconductor trivalent impurity atoms like boron, aluminium, Indium replace atom of pure silicon or germanium in crystal.on application of electric field an electron from one covalent bond may jumps to hold and a new hole appears at the site of electrons and this require very small amount of energy.

This energy is available at room temperature hence large number of holes are available for electrical conduction. Trivalent impurity accepts an electron from neighbouring atom and hence it is called as acceptor impurity.

The electrical conductivity depends upon level of doping and it is independent of temperature. few covalent bonds may break which generates free electrons and equal number of holes.

In p-type semiconductors majority charge carriers are holes. P type semiconductors are also neutral. Trivalent impurities short of one valance electron but it is also short of one proton ( in it’s core).

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